Cutting tools formed of a diamond-based ultra-high-pressure-sintered material having a composition consisting essentially of 0.1 to 15 wt % of at least one of barium carbonate (hereinafter BaCO.sub.3) and magnesium carbonate (hereinafter MgCO.sub.3), the balance being a diamond forming matrix and having a structure wherein the above-mentioned content of at least one of BaCO.sub.3 and MgCO.sub.3 is distributed finely and uniformly in the diamond matrix and having a theoretical density higher than 99% have been used for the finish-cutting of, for example, WC-based cemented carbides or Ti alloys.
However, in view of the strong demand for a reduction in the amount of labor necessary for cutting work as well as the ever-present need for increased production, cutting speed is ever increasing. Unfortunately,, in finish-cutting, for example, Ti alloys or WC-based cemented carbides using a cutting tool made from the above-described diamond-based ultra-high-pressure-sintered material at high speed, chipping (small breaks) occurs quickly and at the cutting edge, making the usable life of the tool comparatively short.